Accurate reconstruction of phylogenetic trees is crucial for understanding evolutionary relationships among organisms. However, the Cambrian Explosion, which saw the sudden emergence of diverse animal phyla and body plans, presents significant challenges for deciphering deep metazoan phylogenetic relationships.
In particular, the molluscan phylum, the second largest phylum in the animal kingdom, poses difficulties due to its diverse fossil record, wide morphological disparity among its eight living classes, and conflicting hypotheses based on paleontological, morphological, and molecular evidence.
To address this, a collaborative team of researchers from China, the U.S., and the U.K. sequenced the genomes of tusk shells, or Scaphopoda, which are enigmatic and poorly studied members of the Mollusca phylum. Their findings, published in the Proceedings of the National Academy of Sciences, reveal ancient incomplete lineage sorting (ILS) during early molluscan evolution.
By considering the impact of ancient ILS, the researchers resolved the long-standing debate surrounding the placement of Scaphopoda in the molluscan tree. Their analysis confirms that Scaphopoda is the sister lineage of Bivalvia, supporting the Diasoma concept proposed based on morphology five decades ago.
Furthermore, this discovery prompts a re-interpretation of important Cambrian fossils that exhibit characteristics of both bivalves and scaphopods. The researchers suggest that certain fossils, such as Anabarella, Watsonella, and Mellopegma, are stem diasomes, revealing insights into the evolutionary origins of these unique body plans.
Dr. Song Hao, from the Institute of Oceanology of the Chinese Academy of Sciences, emphasizes the significance of their work in revolutionizing the understanding of early molluscan evolution.
This study highlights the under-appreciated impact of ancient ILS in reconstructing radiations during the Cambrian period and calls for further exploration of ILS in other challenging nodes of deep metazoan phylogeny.
– Song, Hao et al, Scaphopoda is the sister taxon to Bivalvia: Evidence of ancient incomplete lineage sorting, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2302361120